There are waste-heat recovery systems known from Japanese Patent Application Laid-open Nos.60-93110 and 8-68318, each of which includes heat exchangers mounted at locations upstream and downstream of an exhaust emission control device mounted in an exhaust passage for an internal combustion engine, so that water as a working medium is supplied to the heat exchangers where it is subjected to a heat exchange with an exhaust gas. There are also Rankine cycle systems known from Japanese Patent No.2650660, each of which including an evaporator, an expander, a condenser and a supply pump, wherein a mixture of a medium having a higher boiling point and a medium having a lower boiling point is used as a working medium.
In an internal combustion engine including an exhaust emission control device mounted in an exhaust passage, a catalyst in the exhaust emission control device is not activated in a lower-temperature state immediately after the start of the internal combustion engine and hence, the exhaust emission control performance is temporarily degraded. For this reason, it is required that the catalyst is heated and activated promptly by heat of an exhaust gas. However, if an evaporator in a Rankine cycle system is disposed at a location upstream of the exhaust emission control device, the following problem is encountered: the exhaust gas is robbed of its heat by the evaporator and hence, the activation of the catalyst is retarded. If an evaporator is also disposed at a location downstream of the exhaust emission control device in addition to the location upstream of the exhaust emission control device, then the exhaust gas is robbed of its heat immediately after the start of the internal combustion engine by the upstream evaporator and the exhaust emission control device. For this reason, the downstream evaporator cannot generate a sufficient amount of vapor and thus, it is difficult to effectively operate the Rankine cycle system.